It has long been the goal of naval architects to design and construct vessels with large cargo capacities and internal accommodations, structural strength, stability and steadiness when the vessel is afloat and sufficiently small resistance to economize propelling power as evidenced by U.S. Pat. No. 145,347.
Traditional surface ship monohull designs have usually been developed from established design principles and assumptions which concern the interrelationships of speed, stability and seakeeping. Sacrifices are made to achieve desired performance factors. As a result, current practical monohull surface ship improvements are essentially stalled.
For example, a major limitation of present day displacement hulls is that, for a given size (in terms of displacement or volume), their seaworthiness and stability are reduced as they are "stretched" to a greater length in order to increase maximum practical speed.
Traditional hull designs inherently limit the speed with which large ships can traverse the ocean because of the drag rise which occurs at a speed of about 1.2 times the square root of the ship's length (in feet). For example, a mid-size cargo ship has a top speed of about 20 knots. In order to achieve higher speeds with commercial loads, it is necessary to increase ship length and size (or volume) in proportion, or to increase length while reducing beam, to maintain the same size and volume, but at the expense of stability. Naval architects have long considered the problem of achieving significantly higher ship speeds without increasing length or decreasing beam as the equivalent of "breaking the sound barrier" in aeronautical technology.
Increased length is required for higher speed (except in the case of very narrow hulls which are not practical cargo carriers due to limitations of volume and stability) because of the huge drag rise which occurs at a speed corresponding to a Froude Number of 0.4. The Froude number is defined by the relationship 0.298 ##EQU1## where V is the speed of the ship in knots and L is the waterline length of the ship in feet. To go faster the ship must be made longer, thus pushing the onset of this drag rise up to a higher speed. As length is increased for the same volume, however, the ship becomes narrower, stability is sacrificed, and it is subject to greater stress, resulting in a structure which must be proportionately lighter and stronger (and more costly) if structural weight is not to become excessive. In addition, while for a given displacement the longer ship will be able to achieve higher speeds, the natural longitudinal vibration frequency is lowered and seakeeping degraded in high or adverse sea states as compared to a shorter, more compact ship.
There is an increasing need for surface ships that can transit oceans with greater speed, i.e. in the range of forty to fifty knots, and with high stability because of the commercial requirements for rapid and safe ocean transits of perishable cargoes, high cost capital goods cargoes whose dimensions and density cannot be accepted for air freight, and other time-sensitive freight, particularly in light of the increasing worldwide acceptance of "just-in-time" inventory and stocking practices.
Today, the maximum practical speed of displacement ships is about 32 to 35 knots. This can be achieved in a relatively small ship by making it long, narrow and light but also costly. To some extent it has been possible to avoid increased length above Froude numbers of 0.4, but this has been achieved in small craft design using semi-planing hulls for ships up to 120 feet long and 200 tons and improved propulsion units. In a larger ship, such as a fast ocean liner, the greater length allows a greater size and volume to be carried at the same speed which is, however, lower relative to its Froude number (i.e., 38 knots for an aircraft carrier of 1,100 feet waterline length is only a Froude number of 0.34). On the negative side, the larger size of these ships requires significantly larger quantities of propulsion power. There are major problems in delivering this power efficiently through conventional propellers due to cavitation problems and using conventional diesel or steam machinery which provide a very poor power/weight ratio.
Another means to achieve high speed ships is the planing hull. This popular design is limited to a very short hull form, i.e. typically no more than 100 feet and 100 tons. Boats of only 50 foot length are able to achieve speeds of over 60 knots (or a Froude number of 2.49). This is possible because the power available simply pushes the boat up onto the surface of the water where it aquaplanes across the waves, thus eliminating the huge drag rise which prohibits a pure displacement boat from going more than about 12 knots on the same length of hull. However, at intermediate speeds of say 5 to 25 knots, before the boat "gets onto the plane", a disproportionately large amount of power is required. If a 50 foot boat is scaled to the length of a frigate of 300 feet, the speed scales to the precise range of 12 to 60 knots. Thus scaled, the power required for a 300 foot planing frigate would be about half a million horsepower. Furthermore, the ensuing ride on this 300 foot ship would cause material fatigue as its large flat hull surface would be slammed at continuously high speed into the ocean waves inasmuch as it would be too slow to plane or "fly" across the waves as a much smaller planing ship would do.
Craft utilizing planing hulls have also been produced with waterjet propulsion. Due to limitations of size, tonnage and required horsepower, however, the use of a waterjet propelled planing hull vessel for craft over a certain waterline length or tonnage have not been seriously considered.
In light of the foregoing, I have concluded that the planing hull of the types shown, for example, in U.S. Pat. No. 3,225,729 does not yield the solution to designing large fast ships. However, if the speed categories in relation to waterline length shown in FIG. 13 herein are examined, the semi-planing hull appears to offer attractive opportunities for fast sealift ships. FIG. 13 described hereinbelow shows a continuum of sizes of semi-planing hulls, small to very large. The monohull fast sealift (MFS) hull or semi-planing monohull (SPMH) design is the hull form which is widely used today in smaller semi-planing ships because it offers the possibility of using waterline lengths approaching that of displacement hulls and maximum speeds approaching that of planing hulls.
Hull designs using the concept of hydrodynamic lift are known with regard to smaller ships, e.g. below 200 feet or 200 tons powered by conventional propeller drives as shown in U.S. Pat. No. 4,649,581. The shape of such a hull is such that high pressure is induced under the hull in an area having a specific shape to provide hydrodynamic lift. The MFS or SPMH ship develops hydrodynamic lift above a certain threshold speed as a result of the presence of high pressure at the aft part of the hull. Such a hull reduces the residuary resistance of the hull in water as shown in FIGS. 11 and 14 described below. Therefore, power and fuel requirements are decreased. Since hydrodynamic lift increases as the square of the velocity, a lifting hull allows higher speeds to be achieved. Working boats utilizing the MFS hull or SPMH form are now being used at sea or in many of the world's harbour approaches. This hull form has also up to now been considered limited to certain size fast pilot boats, police launches, rescue launches and fast lifeboats, custom launches, patrol boats, and even motor yachts and fast fishing boats which range in size from 16 to 200 feet (from 2 to about 600 tons). For their size, these boats are much heavier and sturdier than the planing boats. In the speed range of 5 to 25 knots, they have a much smoother ride. They also use much less power for their size at Froude numbers lower than 3.0 than does the planing hull, and they are very maneuverable. However, it has generally been accepted that the practical use of this type of hull is limited to a ship of 200 tons.
FIG. 11 shows a shaft horsepower comparison between an MFS or SPMH frigate (curve A with the circle data points) and a traditional frigate hull (curve B with the triangular data points) of the same length/beam ratio and 3400 tons displacement. Between about 15 and approximately 29 knots both ships require similar power. From 38 up to 60 knots the MFS ship would operate within the area of its greatest efficiency and benefit increasingly from hydrodynamic lift. This speed range would be largely beyond the practicability for a traditional displacement hull unless the length of a displacement hull was increased substantially in order to reduce Froude numbers or the length to beam ratios were substantially increased. Hydrodynamic lift in an MFS or SPMH design is a gentler process which is more akin to a high speed performance sailing boat than the planing hull which is raised onto the plane largely by brute force. An MFS or SPMH hull does not fully plane and thereby avoids the problem of slamming against waves at high speeds.
In addition, modern large ships have traditionally been propeller driven with diesel power. Propellers are, however, inherently limited in size, and they also present cavitation and vibration problems. It is generally recognized that applying state-of-the-art technology, 60,000 horsepower is about the upper limit, per shaft, for conventional fixed pitch propellers. Moreover, diesel engines sized to produce the necessary power for higher speeds would be impractical because of weight, size, cost and fuel consumption considerations.
Waterjet propulsion systems which substantially reduce the cavitation and vibration problem of propeller drives are known as shown in U.S. Pat. Nos. 2,570 595; 3,342,0 3,776,168; 3,911,846; 3,995,575; 4,004,542; 4,611,999; 4,631,032; 4,713,027; and 4,718,870. To date they have not been perceived as useful for propelling larger ships, particularly at high speeds, and are deemed generally too inefficient because they require high pressure at the water inlet in the aft part of the submerged hull, rather than low pressure which generally exists at that portion of large displacement hulls.